Co-pending application Ser. No. 773,532 filed Sept. 9, 1985 in the name of the inventor herein, and assigned to a common Assignee, teaches an indirect fired oven system which utilizes a balance air fan and an exhaust fan. An indraft is induced at the entrance and exit ends of the oven line by the balance air fan, which captures the indraft air at both ends almost immediately that it enters the oven line. Part of the indraft air is fed to a heat exchanger and thence back to the oven line, and the other part of the indraft air is fed back directly and unheated to the oven line. The temperature in the oven zones is controlled by the influx to each of the oven zones of heated air, under control of dampers. An exhaust fan exhausts the oven zones, and that includes taking the volatile solvent which is released from the coating material as it heats up in the oven. The output from the exhaust fan is fed to an afterburner for combustion, following which it flows past the heat exchanger and thence to an exhaust stack. No products of combustion are therefore fed directly to the oven line, or are in contact with the coated product as it moves through the oven.
However, it may sometimes occur that the oven line is, itself, fairly short, so that utilization of the full length or extent of the oven zone or zones for heating and curing the coated product is desirable. In heating and curing the coated product, it is desirable to ensure that the coated product itself --especially when the coated product is such as coiled strip metal--is heated to an optimum temperature at which the coating material is best cured; and it is often desired to maintain the sheet material at that optimum curing temperature. Because of the short period of time in which the coated product is within the oven, the oven zones are maintained at relatively high temperatures (500.degree. F. or more), and various zones of the oven line may be maintained at differing temperatures depending on the purpose intended--such as solvent release, strip metal heating, or dwell--as the coated product travels through the oven line and at any point along the oven line. However, because the coating material contains a volatile and combustible solvent, which may be released from the coated material very rapidly, care must be taken that whatever volatile solvent is released in a gaseous form is captured.
Moreover, it may occur that if the volatile material is not totally captured, particularly at the entrance end of the oven line, where cold intake air is flowing, the volatile solvent may condense on cold surfaces.
It will sometimes occur that a shroud or guide hood is arranged at the entrance end of an oven line, and very often the volatile solvent will condense on the underside of that hood or shroud. When that happens, the condensate may drip onto the coated product as it is being directed into the oven, at which time it may cause blistering, or in the absence of coated product such as at change-over time or otherwise when the oven line is down, the condensate may drip onto the floor in front of the entrance end to the oven. Since the floor may very often be a concrete floor, the volatile solvent dripping onto it may cause severe pitting and other damage.
Another problem may occur when the entrance end of the oven is too cool, or when the coated product such as strip metal has to travel too far from the place where the coating is applied to the entrance of the oven, and that problem is that the coating material may begin to skin. That is, the outer surface of the coating material may harden and form a skin or membrane before the coating material is cured underneath the skin. When that happens, release of solvent from the coating material may occur at the wrong places in the oven, or too slowly; and it may also occur that the solvent may release violently, or it may boil through or past the skin that has set up, and therefore may cause further blistering and a poorly coated product.
Ideally, therefore, the material of the coated product should be heated before the solvent releasing coating material on it is heated. This is especially true, of course, and it is possible, when the coated product is metal. Moreover, because of the high speeds and short time periods that are permitted, such condition is most particularly ideal when coated coiled metal sheet such as aluminum or steel sheet is to be cured.
In order to achieve that condition, however, it is necessary that the coated product be radiantly heated. That is, the metal substrate beneath the solvent releasing coating material should be heated, rather than the coating material itself; and that can only occur in the absence of convection heating, so that radiant heating would be required.
The present invention provides a means whereby the coated product may not only be pre-heated, it may be radiantly heated so as to heat the metal substrate. Moreover, the present invention provides a means whereby better zone heating, particularly in the first zone, is achieved because less cold air enters the first zone of an oven line embodying the present invention. Still further, the present invention provides a means by which the likelihood of solvent condensing at or in front of the entrance to the oven is substantially reduced, if not precluded.
These achievements are gained by providing means for capturing substantially all the indraft air which enters the oven line at or near the entrance port to the oven line, which air is then circulated by a balance fan in at least a first volume to a heat exchanger, from which it is returned back to the oven line. The returning heated balance air is, in keeping with the present invention, at least in part directed to a snout which extends away from the entrance end of the oven line in a direction towards the flow of coated product as it moves towards the entrance port of the line, and the snout has at least one hollow duct which receives that portion of the heated indraft air directed towards it. The heated indraft air is then discharged into the oven line.
The structure of the present invention thereby provides a source of radiant heat, by which the coated product substrate may be heated without heating the coating material itself.